CLUSTERS AT HALF HUBBLE TIME: GALAXY STRUCTURE AND COLORS

We study the photometric and structural properties of spectroscopically confirmed members in the two massive X-ray–selected z 0:83 galaxy clustersMS 1054–03 and RX J0152.71357 using three-band mosaic imaging with the Hubble Space Telescope Advanced Camera for Surveys. The samples include 105 and 140 members of RX J0152.71357 and MS 1054–03, respectively, to i775 < 24:0. A quantitative classification scheme using Sérsic indices and ‘‘bumpiness’ ’ (the ratio of the rms residuals to the model mean) was found to correlate well with visual morphological type. The color-magnitude residuals correlate well with the local density, as measured from both galaxy numbers and weak lensing. Weaker correlations are found with cluster radius (the fundamental dependence is on local density).We identify a threshold surfacemass density of 0:1, in units of the critical density, abovewhich there are relatively few blue (star-forming) galaxies. In RX J0152.71357, there is an offset of 0:006 0:002 in the mean redshifts of the early- and late-type galaxies, possibly from an infalling foreground association of late-type galaxies. A range of star-formation time-scales are needed to reproduce the galaxy loci in the color-color diagrams. We find rest-frame scatters for the ellipticals of 0:03 0:01mag inU B and 0:07 0:01mag inU V. The mean colors and their scatters imply ages 3.5 Gyr, or formation at z 2:2. Thus, when the universe was half its present age, cluster ellipticals were about half the age of the universe; the same is coincidentally true of the median ages of ellipticals today. However, the most massive local ellipticals have agesk10 Gyr, consistent with our results for their likely progenitors at zk 0:8